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Message Posted: Mon, 26 Sep 2005 @ 17:26:52 GMT


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Subj:		Re: Redestributing large tables instead of replicating a small table

From:		Dieter Noeth

Madhukar Nomula wrote:

I know this question is asked before, but I did not see the answer.

I am trying to join a large 40 million row table with a small 2600 row table. But the explain plan is re-distributing the large table instead replicating the small table on all amps.

Here is the query:

          > SELECT * FROM
          > CONTACT C
          > LEFT JOIN
          > LOCATION D
          > ON
          >   C.LOC_ID = D.LOC_ID;

The LOC_ID is the NUPI on LOCATION table.

The optimizer sometimes seems to "forget" about other possibilities if there'a a PI :-)

To make him forget about the PI simply use a calculation on the join column:

     SELECT * FROM
     CONTACT C
     LEFT JOIN
     LOCATION D
     ON
        C.LOC_ID + 0 = D.LOC_ID;

or if it's a char:

     C.LOC_ID || '' = D.LOC_ID;

or for any datatype:

     coalesce(C.LOC_ID, C.LOC_ID) = D.LOC_ID;

This probably changes the explain, then compare explains and performance and please post the results.

Dieter


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